Children with an eye removed offer unique opportunities for studying the role of binocular competition in the development of various visual functions. At the Hospital for Sick Children in Toronto, Canada, we have a unique population of children who were enucleated at an early age for reasons of malignancy. We propose three areas of research that take advantage of the existence of this group: The first study concerns adjustments to localizing objects and obstacles in space and requires measurement of the egocenter (or cyclopean eye), the origin for these localizing judgments. The second series of studies concerns the use of depth cues remaining in these monocular children. Specifically, we will be looking at the use and effectiveness of monocular motion parallax. The third series of studies concerns the measurement of visual functions that may have been altered by neuronal responses to the early enucleation. Specifically, we will be looking at vernier acuity, nasal-temporal differences in acuity, and "oblique effects" in the remaining eye. Because we have access to a relatively large population of children who have been enucleated at ages ranging from 0 to 48 months we believe it will be possible to determine if critical periods exist for some of these visual functions. The development of egocentric (head-centered, as opposed to oculocentric or eye-centered) localization may require some months of binocular experience in order to occur. Similarly, for acuity and oblique effects, there may be evidence that a period of binocularity is required for normal development. We also expect to derive very specific information about what movements of the head are most effective in generating useful motion parallax cues to depth. These could easily be taught to monocular children and adults and would serve to make their orienting and localizing movements in space (and their obstacle avoidance) more accurate.